Staff: Mentor

Hi Ranger, PIR = Passive Infrared. That kind of motion sensor just uses the ambient IR energy in the environment, and looks for changes in the IR imagery. The more complex ones would use an IR camera, and watch for movement in the 2-D IR image. The simpler ones like those used in security systems use a segmented IR lens to focus multiple images onto a single IR sensor. As the IR image changes from motion, the multiple images that are focused on the IR sensor move, and this usually results in a change in the amplitude of the output of the IR sensor. The detector circuitry looks for changes in output voltage greater than some allowed amount per unit time. If you move around slowly enough, you can walk past most PIR sensors. But from my experiments, you have to move reeeeeaallllly slowly!

Okay thanks for the input. I've decided to go with an Active IR motion Dector.

This has a detector and a receiver circuit. As I understand, the beam that is transmitted by an infrared diode is received by an infrared transistor which sets off the alarm. So these are two different circuits that need to be made separately. Isnt there a way of combining the transmitter and receiver into one circuit? If not, do they have to be placed close to each other or something?